(1) Field of the Invention
In underground mining, owing to the dangers of fire-damp and explosions, intrinsically safe designs are exclusively employed for control and regulating current supplies. The permitted voltage and current values in intrinsically safe current supplies lie below the least spark energy for the applicable explosion or fire-damp endangered regions. In mining operations the alternating voltage of the supply network is transformed into an intrinsically safe direct voltage of, for instance, 12 volts and fed into a following intrinsically safe current feed. Onto this intrinsically safe current supply a number of electrical control units are connected, with which for instance electro hydraulic support frame controllers of advancing support frame units are connected, which are employed to support the mining area in underground workings. In such electro-hydraulic operated installations the switching currents available for the initiation of movement of the control values of the hydraulic props and the advancing pistons is only small. A faulty operating current can cause the undesired stopping or release of one or more actuators of the electro hydraulic support controllers, from which functional disturbances in automatic supports and/or the endangering of the safety of the underground personnel can result. A number of switching faults can lead to a failure of the entire support installation.
(b) Description of Related Art
It is proposed in DE-A-3901277 that simple earth leakage can lead to functional faults or a complete failure of the electronic control. In DE-A-3901277 therefore in order to ensure a largely failure-free operation of electronic controllers in intrinsically safe installations, a circuit arrangement is proposed for testing for earth leakage which comprises a test sensor which can be remotely operated and periodically forms a test circuit with a current testing device switched in using an interval switch periodically between the one intrinsically safe potential of the current feed and the protective conductor or the other intrinsically safe potential of the current feed and the protective conductor. Both measurement circuits have a high value control resistance, which can be switched in for a control measurement between the second potential and the protective conductor so as to simulate an artificial earth leakage between the potentials and to be able to prove the satisfactory functioning of the test instruments in the measurement circuit. At intervals using both test circuits even during the operation of the underground installation, a simple earth leakage between one of the potentials and the protective conductor can be indicated. The earth leakage test sensor forms hereby two test circuits separated from each other and at any time an earth leakage can only be established between the potential of the active test circuit and the protective conductor, whilst in the period of time of this measurement an earth leakage from the other potential cannot be established. In particular transient earth leakages can therefore not be detected with the previously proposed circuit arrangement.
The requirements for and the permitted measurements in intrinsically safe current circuits are laid down in the standard EN 50020. The standard includes inter alia a condition that the installation between one intrinsically safe circuit and parts, which can be earthed, must be able to withstand the effective value of a test alternating voltage of 2×500 volts. For reasons of protection against explosion, no potential difference measuring intrinsically safe earth leakage test equipment can be permanently installed in the current feed of an intrinsically safe installation.